Elsevier

Building and Environment

Volume 146, December 2018, Pages 206-215
Building and Environment

Economic and environmental sustainability and public perceptions of rooftop farm versus extensive garden

https://doi.org/10.1016/j.buildenv.2018.09.046Get rights and content

Highlights

  • Use of green roofs as rooftop farm versus extensive garden is evaluated.

  • The life cycle cost of roof garden is lower than flat roof, but roof farm was higher.

  • Lightweight soil is the main economic and environmental contributor of roof farm.

  • A survey study showed that 79.3% of respondents preferred roof gardens over farms.

  • Economic and environmental sustainability of roof farms should be further improved.

Abstract

Green roofs have become popular in urban areas as a solution to restore green space in cities and mitigate urban problems. In this study, the economic and environmental sustainability of using green roofs for rooftop agriculture (i.e., roof farms) is evaluated and compared with that of using green roofs as extensive gardens of flowers and non-edible plants with low maintenance (i.e., roof gardens) based on these two green roofs that were installed and operated for over five years in a university building in Seoul, Korea. The life cycle cost analysis results show that the total cost of the roof garden is 38.9% lower than the flat roof whereas the total cost of the roof farm is 68.3% higher than the roof garden. The environmental impacts of both the roof garden and farm were 2.4–35 times as high as those of the flat roof. The need to frequently replenish the lightweight soil over its lifetime was the main contributor to both the economic cost and environmental impacts of the roof farm, suggesting a need to develop cost-effective and environmentally benign lightweight soil materials. A survey was also conducted to investigate public preferences and perceptions of these two green roof options. Over 80% of the respondents expressed the necessity for green roofs in urban areas, and 79.3% preferred roof gardens over farms. Our results show that roof farms have several merits in urban areas, especially social benefits, but future research should focus on improving their economic and environmental sustainability.

Introduction

Over the past several decades, urbanization has led to the continuous development of infrastructure in cities, and high population densities result in increased demand for energy, water, food, and other resources. With the decreased availability of natural ecosystems, highly dense cities face multiple environmental problems including poor air quality, high volumes of contaminated stormwater runoff, and the loss of natural habitats. Additionally, some cities also experience increased urban temperatures, also known as the urban heat island effect, leading to further increases in energy consumption. These problems will only become more severe as the urban population is projected to increase by a further 3 billion people by 2050 [1].

Green roofs have gained popularity among developers, architects, engineers, and city planners as a sustainable method to aid in resolving urban problems and restore green space in cities. Green roofs consist of plants, a lightweight soil media layer, a waterproofing membrane layer, and a drainage layer on the top of the building's roof, and offer various benefits, such as stormwater runoff reduction [2,3], urban heat island mitigation [4], and an increase in biodiversity [5]. Furthermore, several studies have reported energy savings in the building due to the reduction in the total energy required for air conditioning [[6], [7], [8]] as well as lower environmental footprints due to the reduction in carbon and other pollutant emissions [9].

In addition to the aforementioned environmental benefits, green roofs provide social benefits in cities. Both physical and mental health problems are reportedly linked to a lack of public spaces in cities [10,11]. These negative effects on human health can be minimized with the implementation green roofs, as they improve the availability and access to green space in urban areas [12]. The addition of aesthetic value to the urban landscape has also been suggested as a benefit of green roof installation, although the type of vegetation influenced the extent to which people approved of the visual appearance of the green roofs [13,14].

The incorporation of green roof technology into urban agriculture has also received some attention [15]. With rapid and continuous urbanization in many countries, urban dwellers are becoming increasingly more vulnerable to the food insecurity risks, especially related to the fresh food availability. Urban agriculture is an attractive method of ensuring food security in highly dense urban areas [16]. Urban agriculture fosters community engagement and reduces the energy associated with food transportation, however, securing space in cities for urban infrastructure is often difficult because land is expensive. Farming on green roofs is an attractive option to solve the space issue for urban agriculture. Furthermore, it also serves as an integrated water-energy-food nexus technology that ensures sustainable agriculture while maximizing the efficiency of rainwater use and minimizing building energy requirements [17].

While the use of green roofs as urban farms is a promising and sustainable solution to urban problems, it is important to determine its potential benefits and trade-offs and compare them to those of other types of green roof (e.g., extensive and intensive garden of flower and non-edible plants) as well as those of more conventional flat roofs (i.e., no green roof). The goal of this study is to evaluate and compare the overall sustainability of three roof options (i.e., rooftop farming, extensive green roof, and flat roof) in urban buildings based on real data obtained from two green roofs installed and operated in Seoul, Korea. The economic and environmental costs for each roof option were evaluated using life cycle cost (LCC) analysis and life cycle assessment (LCA). Furthermore, a survey was conducted to identify which green roof type is preferred by stakeholders and how they perceive the associated trade-offs with economic, environmental, and social costs. The specific objectives of this work are to i) quantify the relative contribution of materials and components to the economic costs and environmental impacts of each option, ii) elucidate the trade-offs of each option from economic and environmental perspectives, iii) understand the key values people associate with green roofs, and iv) identify key areas to improve for the use of rooftop for urban agriculture.

Section snippets

Literature review

Over the past decade, numerous studies have investigated the environmental benefits of green roofs in cities. These green roofs were generally categorized into two types, extensive (thin and light soil medium, only shallow-root plants, low maintenance) and intensive (thicker soil medium, a variety of plants and trees, high maintenance) green roofs. Tam et al. [8] had conducted case studies in Hong Kong to evaluate the thermal insulation from the green roof and observed the indoor temperature

System description

This study examines the economic and environmental costs of two green roof options and no green roof option based on information obtained from an existing green roof on the Civil and Environmental Engineering Building of Seoul National University. The building originally had no green roof (herein referred to as the flat roof), and the green roof was installed in 2012. The original flat roof required a polyurethane waterproofing membrane to be re-coated every five years to prevent water

Economic costs of the roof garden and the roof farm

The life cycle costs of the three roof options (i.e., flat roof, roof garden, and roof farm) are summarized in Table 1. The only contributing component to the total cost of the flat roof is the installation and replacement of the waterproof urethane rubber to prevent water infiltration. The installation of urethane rubber costs KRW 3,780,000, and it needs to be replaced every five years, resulting in a total life cycle cost of KRW 37,047,590 over 40 years. Normalized to the roof area, the

Discussion

The life cycle cost analysis results (shown in subsection 4.1) show that the cost of installing a roof garden is lower than that of keeping a flat roof. While the initial cost of constructing a green roof is quite high, the low operation and maintenance requirement results in the overall life cycle cost of the roof garden to be ca. 67% of the flat roof. Wong et al. [26] also stated that extensive green roofs have lower life cycle costs than a conventional flat roof, even when the economic

Conclusions

In summary, the sustainability of two green roof options, a garden and a farm, and the conventional flat roof were evaluated from economic, environmental, and social perspectives. The results of life cycle cost analysis show that the life cycle cost of the roof farm is 176% higher than that of the roof garden, which had a 38.9% lower life cycle cost than the flat roof. The LCA results showed that both green roof options have higher environmental impacts than the flat roof. The results of this

Acknowledgement

This study was financially supported by the National Research Foundation of Korea (NRF-2017R1C1B1003353) and Korea Environmental Industry and Technology Institute (KEITI-2016000200008). It is also supported by the BK21 PLUS research program of the National Research Foundation of Korea.

References (43)

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1

These authors contributed equally to this work.

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